There are several malicious programs that will hijack your web browser and alter your hompage.  In many cases, your anti-spyware program will not remove all of the components of this hijacker and it is likely that it will re-install itself or even damage your TCP/IP stack. If you are using Internet Explorer as your browser, one of the easiest ways to prevent this is to use the advanced functionality of  Spybot Search and Destroy. Go to the Mode tab and select advanced which will then add several additional features. Then go to the 'TOOLS' menu on the left hand panel and select 'IE TWEAKS' - click the box that says "Lock host file read-only as protection against hijackers". This will help prevent your browser from being hijacked.

A word of warning here - Be careful how you use the advanced features of Spybot S&D and make sure you understand what you are doing.